1. Field of the Invention
The present invention relates to an active matrix substrate and a method for fabricating the same. More particularly, the present invention relates to an active matrix substrate provided with an interlayer insulating film having first and second layers containing a specific compound respectively, and a method for fabricating the same.
2. Description of the Related Art
Conventionally, a liquid crystal display device has been used for a variety of applications (e.g., a plane display for a television set, a personal computer, office automation (OA) equipment and the like). An active matrix type liquid crystal display device is known as the liquid crystal display device. The active matrix type liquid crystal display device includes a pair of substrates opposing each other and a liquid crystal layer interposed therebetween, one of the substrates being provided with switching elements such as thin film transistors (TFTs) and an electric field independently applied to the liquid crystal layer in each pixel.
FIG. 5 is a schematic view illustrating an exemplary configuration of a conventional active matrix substrate used for the active matrix type liquid crystal display device.
The active matrix substrate includes TFTs 23 as switching elements and pixel capacitors 22 in a matrix shape. Gate bus lines 24, which function as scanning lines and to control the TFTs 23, are connected to the gate electrodes of the TFTs 23 so as to drive the TFTs. Source bus lines 26, which function as signal lines and input signals to the TFTs 23, are connected to the source electrodes of the TFTs 23 so as to input video signals (data signals) to the TFTs. The gate bus lines 24 (which run parallel with one another) and the source bus lines 26 (which run parallel with one another) cross each other at substantially right angles. One of the pair of terminals of each pixel capacitor 22 is connected with the drain electrode of a respective TFT 23. The other terminal of the pixel capacitor 22 is connected with pixel capacitor bus line 25 so as to be connected with a counter electrode provided on a counter substrate. A liquid crystal display device includes the active matrix substrate and the counter substrate opposing each other, and a liquid crystal layer interposed therebetween including liquid crystal as a display medium.
Referring now to FIGS. 6 to 8, a liquid crystal display device using the above-mentioned active matrix substrate will be described in detail. FIG. 6 is a schematic plan view of the liquid crystal display device. FIG. 7 is a cross-sectional view taken along line VII--VII of FIG. 6, and FIG. 8 is a cross-sectional view taken along line VIII--VIII of FIG. 6.
The active matrix substrate includes a plurality of gate bus lines 24 running parallel with one another and a plurality of source bus lines 26 running parallel with one another. The source bus lines 26 cross the gate bus lines 24 at substantially right angles on a transparent insulating substrate 1 (FIGS. 7 and 8). A pixel electrode 11 is provided for each rectangular region (surrounded by the adjacent gate bus lines 24 and the adjacent source bus lines 26) so as to form a matrix of the pixel electrodes. A branch extends from the gate bus line 24 at each region where the pixel electrode 11 is formed, so as to form a gate electrode 2 of a TFT 23 as a switching element. A gate insulating film 3 (FIGS. 7 and 8) is formed so as to cover the gate electrodes 2, the gate bus lines 24, and the source bus lines 26.
Turning to FIG. 8, each TFT 23 is essentially composed of the gate electrode 2, the gate insulating film 3 covering the gate electrode 2, a semiconductor layer 4 formed on the portion of the gate insulating film 3 corresponding to the gate electrode 2, a channel protecting layer 5 formed on the center portion of the semiconductor layer 4, n.sup.+ -Si layers 6a and 6b formed to cover both sides of the channel protecting layer 5 and the exposed portions of the semiconductor layer 4, an ITO film 7a and a metal layer 8a sequentially formed on the n.sup.+ -Si layer 6a, and an ITO film 7b and a metal layer 8b sequentially formed on the n.sup.+ -Si layer 6b. The n.sup.+ -Si layers 6a and 6b function as a source electrode and a drain electrode of each TFT 23, respectively. The ITO film 7a and the metal layer 8a function as the source bus lines 26 in combination. The ITO film 7b and the metal layer 8b function as a connecting electrode in combination, so as to connect the drain electrode of each TFT 23 with each pixel electrode 11. The ITO film 7b extends to the portion above auxiliary capacitor signal lines 19 (FIG. 7) and an overlapped portion of the ITO film 7b, the gate insulating film 3 and the auxiliary capacitor signal lines 19 forms an auxiliary capacitor.
Furthermore, an interlayer insulating film 9 made of an organic insulating material is formed so as to cover the TFTs 23, the gate bus lines 24, and the source bus lines 26. A transparent conductive film 11 as pixel electrodes is formed on the interlayer insulating film 9. Each pixel electrode is electrically connected with the drain electrode 6b of each TFT 23 via a contact hole 10 penetrating the interlayer insulating film 9 and the ITO film 7b. Furthermore, an alignment film 16 is provided on the entire surface of the substrate so as to cover the pixel electrodes 11.
A liquid crystal display device 700 includes the active matrix substrate and the counter substrate opposing each other, and a liquid crystal layer 17 interposed therebetween including a liquid crystal as a display medium. The counter substrate can be a color filter substrate provided with a light shielding plate 13, a color filter 14, a counter electrode 15 and an alignment film 16 on a transparent insulating substrate 12.
The active matrix substrate having the above-mentioned configuration is disclosed in, for example, Japanese Laid-Open Patent Publication No. 58-172685. According to the active matrix substrate, since the interlayer insulating film is provided between the gate and source bus lines and the pixel electrodes, the pixel electrodes can be formed on the portion of the interlayer insulating film corresponding to the portion where the gate and source bus lines are formed. As a result, a liquid crystal display device having a high aperture ratio can be obtained. Furthermore, defects of a liquid crystal alignment can be prevented by shielding electric fields due to the gate and source bus lines with the interlayer insulating film.
However, the interlayer insulating film made of an organic insulating material has the following problems. When the transparent conductive film (e.g., ITO film) as pixel electrodes is formed directly on the interlayer insulating film made of an organic insulating material, separation between the ITO film and the interlayer insulating film is likely to result. This is because adhesion of the interlayer insulating film to the ITO film is insufficient. The separation between the films causes reduction of yield of the active matrix substrate. Consequently, the fabrication cost of the active matrix substrate is increased thereby making the liquid crystal display device using such an active matrix substrate expensive.
In order to overcome the above-mentioned problems, an active matrix substrate using a combination of an organic insulating film and an inorganic insulating film as an interlayer insulating film has been proposed in Japanese Laid-Open Patent Publication No. 4-163528. According to the active matrix substrate disclosed therein, since the inorganic insulating film is provided between the organic insulating film and the ITO film, separation between the ITO film and the interlayer insulating film can be prevented.
However, the fabrication of such an active matrix substrate is considerably complicated. In particular, the fabrication of the active matrix substrate requires independently forming the organic and inorganic films and independently forming contact holes in the organic and inorganic films. Therefore, the fabrication of this active matrix substrate requires the steps of forming the organic and inorganic films respectively, and photolithographing and etching the organic and inorganic film so as to form contact holes respectively. In other words, the fabrication of this active matrix substrate requires six steps for forming the interlayer insulating film. As a result, the active matrix substrate still has the problem that a liquid crystal display device using such an active matrix substrate is expensive.
In view of the above-mentioned problems, it is desired to provide an active matrix substrate capable of realizing a liquid crystal display device having a high aperture ratio and a satisfactory liquid crystal alignment, and a method for easily and inexpensively fabricating such an active matrix substrate.